1. Field of the Invention
The present invention relates to a wireless communication system for communication between a wireless base station and a mobile station and to a wireless communication method.
2. Description of the Related Art
An LTE (Long Term Evolution)-based wireless communication system between a wireless base station and a mobile station has been proposed (3GPP TR 25.814 V7.1.0 (2006-09)).
With LTE, a packet switching-type access system is adopted, with wireless resources being assigned using frequency domain scheduling to the uplink (from the mobile station to the wireless base station) and the downlink (from the wireless base station to the mobile station). An LTE system will be described by taking the LTE uplink that appears in 3GPP TR 25.814 V7.1.0 (2006-09) as an example.
FIG. 8 is a conceptual view of the wireless communication system. The wireless communication system has a wireless base station (‘base station’ hereinbelow) 100 and a mobile station 200.
A random access channel, an uplink reference channel, an uplink shared data channel, and an uplink control channel is transmitted from the mobile station 200 to the base station 100 in the uplink.
The random access channel is used at the time of initial access when communication is started or during handover and is utilized in order to establish synchronization between the uplink mobile station 200 and base station 100.
The uplink reference channel is a channel that is used in demodulation processing of shared data channel and control channel, in the measurement of communication quality (channel state such as, for example, the ratio between the desired signal power and undesired signal power (SIR, SNR or the like)) for the purpose of scheduling, and in delay profile measurement for transmission timing control.
The uplink shared data channel is a channel that is used in the transmission of traffic data (information bits) and is transmitted by using wireless resources which is assigned by the scheduling of the base station 100.
The uplink control channel is a channel that is used in order to transmit information that is required for processing to receive the shared data channel (data size and retransmission control information) and to transmit scheduling requests (traffic amount and information type or the like).
However, a downlink control channel is transmitted from the base station 100 to the mobile station 200 in the downlink. The downlink control channel is a channel that is used in order to transmit assigning information of the uplink shared data channel that is assigned by the scheduling in the base station 100 (scheduling information) and to transmit an ACK/NACK result for the uplink shared data channel, or the like.
FIG. 9 shows an example of the uplink frame format (3GPP R1-070266, Texas Instruments, “Summary of Reflector Discussions on EUTRA DM RS”, January 2007). A CP (Cyclic Prefix) to which a rear portion of the symbol data is copied is inserted in a front portion of each symbol. The reference channel or the shared data channel or the like is inserted in the symbol of a predetermined region and transmitted from the mobile station 200 to the base station 100. Further, as shown in FIG. 9, a reference channel for demodulating the shared data channel is inserted in symbols ‘#4’ and ‘#11’, and a frame with a ‘1 ms’ transmission interval is constituted by fourteen symbols.
FIG. 10 represents the uplink frame format in two dimensions which are the time axis (horizontal axis) and frequency axis (vertical axis). The system bandwidth is divided into a plurality of bands, and wireless resource blocks are defined by the divided bandwidth and single block that is delimited by the transmission interval. The uplink scheduling is performed by assigning users to the respective wireless resource blocks. In the example shown in FIG. 10, a control channel is assigned to the wireless resource block at both ends of the system bandwidth and a control data channel is transmitted without being attached to the shared data channel.
FIG. 11 shows a timing example in a case where the signals from the respective users are received in sync by the base station 100. Generally, in the case of the uplink, the wireless resource blocks are transmitted with different timing and, therefore, the timing with which the base station 100 receives the blocks varies. However, the cutting range in cases where the base station 100 subjects the respective symbols to an FFT is fixed. Therefore, the base station 100 uses transmission timing control to control the transmission timing for each user (mobile station 200) so that the fixed FFT cutoff range of the base station 100 lies within the symbol range of each user. The transmission timing control is executed as a result of the transmission timing control information generated on the basis of the delay time that is obtained from the delay profile being transmitted from the base station 100 to the mobile station 200 and as a result of the mobile station 200 adjusting the transmission timing on the basis of this information.
As shown in FIG. 11, reception signals which are in a synchronized state are subjected to FFT which is performed collectively with fixed timing by the base station 100 and are divided into respective wireless resource blocks. However, in cases where a reception signal that is not in a synchronized state exists, the reception signal no longer possesses an orthogonal frequency characteristic after the FFT processing and interferes with the reception signals of other users.
In the case of OFDM, when a reception signal undergoes FFT processing and the spectrum is illustrated, the subcarrier which is adjacent to the NULL point of a certain subcarrier (the point at which the signal power is lowest) is the peak. In this case, the frequency characteristic of the reception signal has an orthogonal relationship and no interference between the respective subcarriers is produced.
However, when the reception signal of a certain user is not contained in the FFT cutoff range, the adjacent subcarrier is not the peak at the NULL point of a certain subcarrier. In this case, interference is produced because the frequency characteristic is not orthogonal.
However, in an LTE uplink, the random access channel (‘RACH’ hereinbelow) is also multiplexed in wireless resource blocks. FIG. 12 shows an example of a RACH multiplexing method. As shown in FIG. 12, the RACH uses six wireless resource blocks and the transmission of the RACH with a ‘10 ms’ transmission interval is determined by 3GPP TR 25.814 V7.1.0 (2006-09) below.
FIG. 13 shows an example of the RACH frame format (3GPP TR 25.814 V7.1.0 (2006-09) hereinbelow). In RACH transmission, a frame is constituted by a CP having a 0.1 ms interval and a RACH having a 0.8 ms interval, and a guard time having a 0.1 ms interval is provided.
A RACH is channel data that are initially transmitted from the mobile station 200 to the base station 100 and is transmitted in a non-synchronization state such as during initial access as mentioned earlier. As a result of the RACH being transmitted in a non-synchronization state, it is difficult to make the channel signals of the other wireless resource blocks and the frequency characteristic orthogonal. However, by providing the RACH with a guard time as shown in FIG. 13, it is possible to prevent the generation of interference with the next transmission interval.
However, even when a guard time is provided, as shown in FIG. 13, when a signal is transmitted with the same transmission timing as the RACH, there is mutual interference between the signal and the RACH.
FIG. 14 schematically represents the interference of the RACH signal with adjacent wireless resource blocks. Normally, the spectrum of the transmission signal is not rectangular and has a shape with a fixed spread. Although the signals of the adjacent resource blocks likewise also has a spread shape, in cases where there is mutual synchronization, the spread components of the spectrum are also orthogonal on the frequency axis and do not interfere with one another. However, the frequency characteristics of signals which are transmitted asynchronously such as RACH signals are not orthogonal and, in cases where a signal has been assigned to the wireless resource blocks which are located right next to the RACH (#m+2, #m+9) in particular, there is the problem that the interference of the RACH is very large and the transmission characteristic of the signal deteriorates over a wide interval.